On 4/20/2011 10:19 PM, Grant Saviers wrote:
> Bottom line, even with the article's promise of a "no tweaks" design,
> you will likely need to prune to get what you want,
I've built, measured, and used a LOT of dipoles in the last seven years,
and I'd call anyone who advertises a "no-tweak design" either a huckster
or a fool! There are simply too many variables -- the dielectric
constant of the wire that you use, surrounding objects, ground
conductivity, and the height above ground among them.
Another important point -- the feedpoint impedance at resonance of a
half wave dipole varies a lot depending on its height above ground. Low
dipoles (a half wave or less) tend to be closer to 50 ohms and high
dipoles tend to be closer to 75 ohms. This simple difference will cause
a shift of 1% or so in the minimum SWR. Further, if you use 75 ohm coax
and measure the SWR with a 50 ohm SWR bridge, you will get wrong answers
for the SWR!
Think about this -- two things about SWR matter. First, the match
between the antenna and the transmission line determines the loss in the
line. Second, the feedline impedance where it connects to the
transmitter output needs to be close to 50 ohms to get max output and
keep the transmitter happy. #2 is the problem that antenna tuners fix.
Further, power amps with a tuned output stage (most of those using
tubes) are able to be happy over a moderately wide range of load
impedance. That is, the tuned output stage acts as an antenna tuner.
Getting back to the original question in this thread about antennas for
160/80 for a DX trip one thing I would not leave home without is a good,
high power antenna tuner!
73, Jim Brown K9YC
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